Recent developments in the phylogeny and classification of Ralstonia solanacearum

Ralstonia solanacearum strains are highly variable and adaptable as attested by worldwide distribution and a large and expanding host range. R. solanacearum (Rs) and its close relatives, the blood disease bacterium and R. syzygii, constitute a species complex, a diverse group of related isolates that represent more than one species. Biovar (Bv) typing and race assessment are methods commonly used for assessing the diversity of Rs strains. However, recent genetic evidence has indicated that these phenotypically-based schemes are not sufficient to encompass the diversity of strains represented in the species Rs. Here we present a classification system based upon phylogenetic analysis of sequence data generated from the 16S-23S internal transcribed spacer (ITS) region, the endoglucanase gene and the mutS gene. From the phylogenetic analysis of the sequence data the Rs species complex can be subdivided into four monophyletic cluster of strains, termed phylotypes. Strains within each phylotype broadly originate from the same geographic location. Phylotype I includes all strains belonging to Bv 3, 4, and 5 and strains are isolated primarily from Asia. Phylotype 11 includes strains belonging to Bv 1, 2 and 2T isolated primarily from America. The Rs race 3 potato pathogen, which has a worldwide distribution, and the race 2 banana pathogen are both members of phylotype II. Phylotype III contains strains primarily isolated from Africa and surrounding islands. Strains in this group belong to Bv 1 and 2T. Phylotype IV contains strains isolated primarily from Indonesia belonging to Bv 1, 2 and 2T. Each phylotype is composed of a number of groups of strains with a highly conserved sequence termed sequevars. Some sequevars contain strains which are pathogenic on the same hosts or strains of common geographic origin. The phylotyping scheme is highly discriminatory, flexible, additive, and should allow better prediction of the properties of strains. This in turn will aid in the successful control of the many bacterial wilts caused by Rs, including bacterial wilt of tomato.